The present invention relates to drain pumps for home appliances, in particular for washing machines, dryers and dishwashers.
Drain pumps for home appliances such as washing machines, dryers and dishwashers comprise a motor with a turning shaft, an impeller connected to said turning shaft and a hydraulic body in which said impeller is housed. Said hydraulic body has an inlet conduit from which the waste water from the washing chamber of the home appliance accesses the drain pump, and an outlet conduit from which said waste water is removed to the drain conduit.
In drain pumps the waste water must be prevented from returning in the opposite direction, passing from the drain conduit to the washing chamber of the home appliance, for which purpose anti-return valves are used.
ES 2142714 B1 discloses a water removing system in a washing machine that includes a drain pump that comprises in the outlet conduit a flap valve to perform the anti-return function. Said flap valve is opened when the water flows towards the drain conduit, but is closed by the action of the waste water itself when said water flows in the opposite direction. DE 3715285 A1, DE 19546967 A1 and EP 1162300 A2 disclose other examples of drain pumps with anti-return valves that are closed by the action of the waste water.
This type of anti-return valve has the drawback of offering no protection against the Venturi effect. Due to the Venturi effect, the passage of the water through the main conduit of the drain pump may cause the suction of the water from the washing chamber of the home appliance, with the anti-return valve not posing any resistance whatsoever to said suction. This means that additional elements must be used to prevent the Venturi effect, for example a siphon trap in the drain installation.
The drawback with these anti-return valves is that they tend to create priming problems in the drain pump, as they can lead to air accumulating inside the hydraulic body of the drain pump, thereby preventing in some cases said drain pump from functioning normally.
Said priming problems are also created in the drain installation. Siphon traps that usually has said installation to prevent the Venturi effect, prevent the passage of waste water thereby creating head loss leading to the accumulation of air.
For other applications, different from drain pumps for home appliances, pumps have been used that comprise a motor comprising actuating means to block one of said conduits when said motor is not being powered, the motor itself performing the function of a shut off valve.
DE 2510787 A1 discloses a circulating pump for central heating that comprises an impeller formed to act as shut off valve when the pump stops. Said pump comprises a motor with a turning shaft, said motor comprising a fixed stator and a rotor linked to said turning shaft, the impeller being connected to said turning shaft. When the motor is not being powered, said impeller acts as shut off valve. When said motor is powered, the magnetic field created by the stator moves the turning shaft axially and the impeller stops acting as shut off valve.
However, the pump of DE 2510787 A1 is, as stated before, for heating systems. In any event, it would be difficult to use a pump with the referred features as drain pump for home appliances, because a greater dimensioning of the stator of the motor is necessary in order to create a magnetic field able to move the turning shaft axially. A considerable increase of the size of the motor of the pump is a serious drawback for drain pumps for home appliances. Moreover, particularly if a permanent-magnet synchronous motor was used, apart from the size increase, there would be an important increase of energy consumption and costs.